The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950° C. is more than one hundred years old.
Conventional aluminium production cells are constructed so that in operation a crust of solidified molten electrolyte forms around the inside of the cell sidewalls. At the top of the cell sidewalls, this crust is extended by a ledge of solidified electrolyte which projects inwards over the top of the molten electrolyte. The solid crust in fact extends over the top of the molten electrolyte between the carbon anodes. To replenish the molten electrolyte with alumina in order to compensate for depletion during electrolysis, this crust is broken periodically at selected locations by means of a crust breaker, fresh alumina being fed through the hole in the crust.
This crust/ledge of solidified electrolyte forms part of the cell's heat dissipation system in view of the need to keep the cell in operation at constant temperature despite changes in operating conditions, as when anodes are replaced, or due to damage/wear to the sidewalls, or due to over-heating or cooling as a result of great fluctuations in the operating conditions. In conventional cells, the crust is used as a means for automatically maintaining a satisfactory thermal balance, because the crust/ledge thickness self-adjusts to compensate for thermic unbalances. If the cell overheats, the crust dissolves partly thereby reducing the thermic insulation, so that more heat is dissipated through the sidewalls leading to cooling of the cell contents. On the other hand, if the cell cools, the crust thickens which increases the thermic insulation, so that less heat is dissipated, leading to heating of the cell contents.
The presence of a crust of solidified electrolyte is considered to be important to achieve satisfactory operation of commercial cells for the production of aluminium on a large scale. In fact, the heat balance is one of the major concerns of cell design and energy consumption, since only about 25% of such energy is used for the production of aluminium. Optimisation of the heat balance is needed to keep the proper bath temperature and heat flow to maintain a frozen electrolyte layer (side ledge) with a proper thickness.
In conventional cells, the major heat losses occur at the sidewalls, the current collector bars and the cathode bottom, which account for about 35%, 8% and 7% of the total heat losses respectively, and considerable attention is paid to providing a correct balance of these losses.
Further losses of 33% occur via the carbon anodes, 10% via the crust and 7% via the deck on the cell sides. This high loss via the anodes is considered inherent in providing the required thermal gradient through the anodes.
In the patent literature, there have been suggestions for cells operating without a crust of solidified electrolyte.
U.S. Pat. No. 5,368,702 (de Nora) discloses a multimonopolar aluminium production cell operating with tubular anodes in a crustless molten electrolyte which is thermally insulated by a cover. The cover is lined underneath with a layer of thermally insulating material. U.S. Pat. No. 5,415,742 (La Camera/Tomaswick/Ray/Ziegler) discloses another aluminium production cell operating with a crustless molten electrolyte which is thermally insulated by a cover.
WO02/06565 (D'Astolfo/Hornack), U.S. publications 2001/0035344 (D'Astolfo/Lazzaro) and 2001/0037946 (D'Astolfo/Moor) disclose an aluminium production cell having thermally insulating cover sections over the cell's electrolyte and several inert anode blocks that are suspended from each cover section, the cover sections serving also to distribute current to the inert anode blocks connected thereto.
U.S. Pat. No. 6,402,928 (de Nora/Sekhar) discloses an aluminium production cell having an insulating cover made of sections associated with individual anodes or groups of anodes, the insulating cover being removable by sections so that the individual anodes or groups of anodes can be separately replaced or serviced by removing only the removable sections associated therewith.
Despite previous efforts to develop a cell design for operation with non-carbon anodes, there is still a need to provide an aluminium production cell with an insulating cell cover permitting simplified cell operation.